The carriage having a print head thereon is generally moved by a pin on a toothed belt and which is engaged in a hole formed in the carriage.
A conventional toothed belt comprises many teeth which are formed at even intervals and a holder for rotatably holding a guide roller. The guide roller is engaged in a long and narrow hole in the carriage which in turn is slidably mounted between two parallel guide rails. When the toothed belt is moved by rotating a moving pulley, the carriage is moved along the guide rails by the moving of the guide roller as it is carried along by the belt. When the guide roller reaches the outside periphery of the pulley, the motion of the guide roller changes from a straight line to a curved motion. The guide roller moves the carriage to the end of the printing line while moving in the hole. When the guide roller moves more than 90 degrees around the pulley, the carriage is moved in the reverse direction.
In such a conventional toothed belt, the guide roller is rotatably mounted on the belt by the holder for preventing contact between the guide roller and the toothed pulleys.
The pitch line of such a toothed belt runs outward off the center axis of the guide roller. Therefore, a moment is produced at the spot where the holder is mounted on the toothed belt. This moment and load variation during printing cause the belt to vibrate. This vibration is transmitted to the carriage and causes disarray of the print. It is especially great at the position where the carriage reverses.